This invention relates generally to determination of the depth of rock formations in the earth having abnormally high pressure (geopressure) conditions therein, and more particularly, to a method for predicting pressures in such formations prior to drilling of a borehole penetrating the formation.
In the process of forming sedimentary rocks, older sediments are buried deeper and deeper, with the overburden weight causing compaction. During compaction, porosity typically decreases, pore fluids are squeezed out, and remaining fluids maintain a hydrostatic pressure gradient with depth. However, if impermeable layers surround regions with pore fluids, entrapped fluids cannot escape and are subjected to above normal pressures (geopressure) within a formation.
During drilling of a borehole, drilling fluids, usually referred to as "muds," are circulated in the borehole to cool and lubricate the drill bit, flush cuttings from the bottom of the hole, carry cuttings to the surface, and balance formation pressures encountered by the borehole. It is desirable to keep rotary drilling mud weights as light as possible to most economically penetrate the earth; heavier muds may break down rocks penetrated by the borehole and thereby cause loss of mud. Mud weight is carefully monitored and may be increased during drilling operations to compensate for geopressure. In some areas, however, there may be unexpected abnormal increases in pressure, with depth such that mud weight does not compensate pressure; the result can be blowout of the well.
The reason that such geopressure conditions occur is that fluids become trapped in rock and must support some of the overburden weight. Also, there may be an earth formation of high porosity and high permeability, or a series of such formations, within a massive shale formation that is relatively impermeable so that fluid pressure is transmitted into such highly permeable formations (which usually are sands) as weight of overburden increases during sedimentation. When such formations are penetrated, the large pressure gradient into the borehole can easily result in a blowout.
It is desirable to set casing in a borehole immediately below top of a geopressured formation and then to increase mud weight for pressure control during further drilling. Setting a casing string which spans normal or low pressure formations permits the use of very heavy drilling muds without risking breaking down of borehole walls and subsequent lost mud. On the other hand, should substitution of heavy drilling mud be delayed until the drill bit has penetrated a permeable overpressurized formation (e.g., sandstone), it may be impossible to remove the drill string without producing a blowout or otherwise losing the well.
In areas where there is reason to suspect existence of such high pressure formations, various techniques have been followed in attempts to locate such geopressure zones. For example, acoustic or electric logs have been run repeatedly after short intervals of borehole have been drilled or are acquired using measurement-while-drilling techniques, and a plot of acoustic velocity or electrical resistance or conductivity as a function of depth has been made. Abnormal variations of acoustic velocity and/or electrical properties obtained by logging may indicate that the borehole has penetrated a zone of increasing formation pressure. It is manifest that such techniques are very expensive and time-consuming and cannot predict what pressures will be encountered ahead of the bit.
Furthermore, oil industry experience with drilling in the Gulf of Mexico indicates that almost 99% of commercial oil/gas fields are found in environments with only "mild" to "moderate" overpressures, and that regions of "hard" or high geopressure rarely contain commercial hydrocarbon accumulations. Therefore, predrill evaluation of the pressure regime of a prospsective hydrocarbon target is critical to economic assessment of that target due to both low probability of economic hydrocarbon reserves in hard geopressure regions and increased total drilling costs in geopressure. Occurrence and timing of geopressure is closely related to sediment deformation and faulting and directly impacts migration of hydrocarbons.
U.S. Pat. No. 3,898,610 (Pennebaker) discloses methods of geopressure assessment in an area proposed for drilling: first, perform a seismic observation (using a common midpoint (CMP) method as illustrated in Pennebaker FIG. 1) to determine average seismic velocity as a function of depth; next, compute interval transit time as a function of depth; and then compare these observed interval transit times to putatively normal interval transit times as illustrated in Pennebaker (FIG. 4). Depths where observed interval transit times are greater than normal indicate lower-than-normal velocity and inferentially greater-than-normal porosity and thus geopressured fluids. Putatively normal interval transit times are either (i) directly measured in a borehole in the general area which encountered only normal pressures during drilling or (ii) computed by following an expression for seismic velocity V (feet/second) as a function of depth D, with D measured in feet from a location of known seismic velocity V.sub.o : EQU Log.sub.10 V(D)=Log.sub.10 V.sub.o +KD;
where K is a constant equal to about 0.000029.
However, Pennebaker has problems with calibration of seismic velocity to well log and drilling information, quantifying computed pressure gradient from seismic transit time, pressure calibration dependence upon geological province and age, and being limited to vertical predictions.
The present invention provides predictions of two-dimensional pressure cross sections which provide for deviated well prediction, use translational curves for quantifying pressure gradient predictions, can be enhanced using vertical seismic profile (VSP) or checkshot information acquired during drilling (e.g., when data indicate that better calibration of seismic data is needed), and provide by the two-dimensional pressure cross sections visualization to understand structural and stratigraphic control of geopressure features and thus take advantage of geological information accumulated by explorationists in an area.